1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means can comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
A lithographic apparatus is known in which a pattern is imparted to a beam by an array of individually controllable elements. Rather than relying upon a preformed mask (also referred to as a reticle) to impart a pattern to a beam, control signals are delivered to the array of controllable elements to control the state of those elements to pattern the beam. This is generally referred to as “maskless” given that it relies upon individually controllable elements rather than a mask to impart the necessary pattern to the beam. A maskless lithographic apparatus can be used to expose relatively large area substrates, for example substrates to be used as flat panel displays. The panels are exposed in a single pass beneath an array of projection systems, each of which is provided with its own patterning system incorporating an array of individually controllable elements. As the substrate is displaced relative to the projection systems, it is necessary to change the state of individual elements in the arrays of controllable elements so as to change the projected patterns. The rate at which the state of the individual elements can be changed, generally referred to as the update rate, is limited and this imposes an upper limit on the maximum speed at which a substrate can be displaced relative to the projection systems. The speed of displacement determines the maximum throughput of the apparatus, and therefore it is desirable to be able to increase the speed of displacement.
It is possible to increase the substrate displacement speed by increasing the number of projection systems devoted to the exposure of a single track of pixels in the substrate scanning direction. For example, a substrate displacement speed can be doubled if two projection systems are arranged in series in the scanning direction. With such an arrangement each adjacent pair of pixels in the scanning direction can be exposed by a respective one of the two projection systems.
The substrate displacement speed can be further improved by adding further rows of projection systems. Three rows of projection systems trebling the maximum speed and four rows quadrupling the maximum speed. Increasing the substrate speed brings with it its own problems however in terms of maintaining appropriate speed control and achieving the necessary acceleration and deceleration of the substrate before and after scanning of the substrate.
Furthermore, adding extra rows of projection systems increases the overall distance that a substrate has to be displaced to achieve a full scan. For example, a row of projection systems capable of exposing the full width (perpendicular to the scanning direction) of the substrate is typically of the order of 100 millimeters deep in the scanning direction and therefore, given a single row of projection systems and a substrate 2 meters long in the scanning direction, a total scan range of 2.1 meters is required. Adding a second row of projection systems increases the scan range to 2.2 meters and so on.
However, adding additional rows of projection systems does not result in a proportionate increase in throughput. This is because as the total area that has to be exposed is also larger, adding one projection system increases the total area by the area of that projection system. In addition, adding rows of projection systems also increases the physical footprint of the apparatus.
Therefore, what is needed is a system and method that increases throughput in a maskless lithography system.